Aberrant apoptosis contributes to various neurodegenerative disorders. Elaborating the signal transduction mechanisms that regulate neuronal survival is thus important for understanding both basic biology and for therapeutic intervention. Neurotrophins potently stimulate neuronal survival in part by activating the small GTP-binding protein Ras, which functions by translating and directing neurotrophin-initiated signals into multiple signaling pathways. Recent data indicate that the Ras-dependent signaling pathways, PI-3 kinase/Akt and MEK/ERK, and the Ras-independent MEK5/ERK5 signaling cascade, are the primary mediators of neurotrophin-dependent survival. We have discovered a novel, evolutionarily conserved group of Ras-related proteins. There are at least two highly related human genes (Rit and Rin), while Drosophila express only one family member (Ric). These proteins represent a gene family that functions to regulate signaling networks that have been conserved from flies to man. Overexpression of a constitutively active mutant of Rit in pheochromocytoma cells induces neurite outgrowth and survival by activating a MEK-dependent, but PI-3 kinase/Akt-independent, signaling pathway. In addition, activated Rit expression in primary neurons inhibits trophic factor-withdrawal induced apoptosis and promotes axonal outgrowth. Thus, Rit controls survival pathway(s) in a manner distinct from that of Ras, likely functioning to control MEK5/ERK5 signaling, or regulating a novel signaling pathway, which activates ERK kinases. We hypothesize that Rit functions to translate and direct neurotrophin-initiated signals to pro-survival signaling cascades in a manner distinct from Ras. Three specific aims are described to evaluate this hypothesis. Specific Aim 1 will determine if activation of Rit signaling pathways promotes the survival of primary cultured neurons and if Rit function is essential for neuronal cell survival. Specific Aim 2 will examine the extracellular stimuli that regulate Rit function, via neurotrophin-associated or neurotrophin-independent pathways. Specific Aim 3 will examine the mechanism of Rit-mediated ERK activation and determine if established ERK-dependent pro-survival signaling pathways are important for Rit-mediated neuronal survival. Preliminary studies have demonstrated that Drosophila is sensitive to altered Ric activity, resulting in developmental defects in the wing and eye. We will undertake a genetic screening approach to identify genes that interact with Ric, and vertebrate homologues will be analyzed for their ability to contribute to Rit function in neuronal cells. Through the combined use of biochemistry, molecular biology, and genetics, these studies will form the foundation for understanding the function of this unique regulator of neuronal survival. [unreadable] [unreadable]